Compound semiconductor materials have proven useful in a variety of applications. One such compound is Mercury Cadmium Telluride (HgCdTe). This compound semiconductor is particularly useful in detection of infrared radiation. Infrared radiation is a nearly universal byproduct of heat generation. Thus, detection of infrared radiation provides the ability to locate heat sources when visible location is not feasible, such as at night, Infrared detection also has numerous applications in law enforcement, military and other public safety fields. HgCdTe is particularly useful for infrared detection because it can be “tuned” to a particular wavelength region of interest. This is accomplished by varying the proportions of Mercury and Cadmiun in the alloy.
However, HgCdTe devices have proven to be particularly difficult to manufacture. To achieve the desired performance, some devices require cooling devices to keep the detector at a temperature that allows for adequate sensitivity in infrared to allow measurable affects. Uncooled detectors have also been developed. These detectors provide acceptable performance without the need for cooling devices. An example of this type of device can be found in Eden, “Uncooled Infrared Detector,” U.S. Pat. No. 6,100,525, which is assigned to the assignee of this application and which is hereby incorporated by reference. Some devices require elaborate and costly manufacturing techniques. Examples of these processes include Eden et al, “Method for Fabricating an Infrared Radiation Detector,” U.S. Pat. No. 6,198,100 B1, which is assigned to the assignee of this application and which is hereby incorporated by reference, and Chandra et al., “Integrated Infrared Detection System,” U.S. Pat. No. 6,091,127. These elaborate processes are required by limitations of HgCdTe crystal structures.
To create an effective detector, it is necessary to form a portion of the HgCdTe crystal with a mercury to cadmium ratio tuned to the selected detection wavelength band. Then a detector area must be isolated. This allows for measurement of electrical effects on the isolated region caused by the absorption of infrared photons. Most prior techniques for isolating detectors require formation of the HgCdTe crystal mesas. The surfaces of the mesas provide a large area for potential surface recombination of carriers, which reduces the performance of the device. In addition, if the substrate is a crystal having a different lattice structure than the mesa, internal strains in the epitaxial layers cause dislocations which limit the performance of the detector. It is therefore desirable to provide a detector structure and manufacturing process that does not include the need for mesa-type structures. Furthermore, the materials ought to have a minimum of dislocations, especially at the layer interface.
For planar geometry, non-mesa detectors, prior manufacturing techniques use ion implantation to form doped regions for detectors. However, implantation causes damage to the crystal structure of the detector that is difficult to anneal away. This reduces the integrity of the crystal structure, which may provide another source of leakage and carrier recombination.